The non-contact measurement of A.C. voltages, by means of measuring the A.C. current caused by capacitive coupling between the unknown wire and a sensing plate, is well established. Most of these methods rely on knowing the capacitance between the wire-under-test and a sensing plate. If this capacitance is not known, the reading will not be accurate, as the current flowing into the sensing plate is proportional to the product of the time derivative of the voltage on the wire and the capacitance between the wire and the sensing plate. One prior invention, U.S. Pat. No. 5,473,244 (Libove and Singer), overcomes the problem of determining the unknown capacitance by dynamically measuring it using an A.C. reference voltage applied to the sensing plate.
Unfortunately, the above measurements are not suitable for non-contact D.C. measurements on a wire, as the derivative of a D.C. voltage is zero, and hence no steady-state current flows into the sensing plate that would allow the measurement device to determine the D.C. voltage on the wire. Conventional methods which do allow non-contacting measurements of D.C. potentials on wires or surfaces generally make use of electrometers, which measure the electrostatic force either between the unknown wire and a charged plate, or between two charged plates that are brought into close proximity to the unknown wire. Such methods are well established, but provide poor accuracy, particularly for low D.C. voltages. Since these methods rely on measuring the mechanical deflection of charged plates, they are sensitive to undesirable error sources such as gravitational effects, temperature effects, D.C. amplifier drift, and susceptibility to low-frequency vibration, making them unsuitable for portable or handheld use. Also, their reading varies with variations in coupling capacitance between the unknown wire and the sensing apparatus. Since this capacitance varies with orientation, wire and sensor geometries, and dielectric constants, the readings are merely proportional to voltage on the unknown wire, rather than conveying an accurate absolute voltage measurement. More importantly, their low sensitivity makes them unsuitable for accurate measurements of voltages below approximately 50 volts D.C., which make up the majority of applications in which D.C. voltage measurement is required, including automotive, aircraft, shipboard, and computer applications. Finally, the size of existing electrometers makes them too large to be used to measure voltages in tight quarters, such as exist in most modern electrical and electronic systems.
The present invention overcomes these problems and results in a system that can accurately measure D.C. and A.C. voltages from the sub-volt level up to many tens of thousands of volts, and provides means for virtually eliminating errors due to capacitance variations, temperature changes, vibration and mechanical orientation between the wire and sensor.
The present invention provides a method and apparatus for performing non-contacting measurements of the voltage, current and power levels of conductive elements. The voltage measurement system of the present invention consists of a conducting membrane or plate that is fastened to the diaphragm of a microphone or other electromechanical, electronic, or electro-optical transducer. The membrane is brought into proximity with the conductive element (wire or other object) whose unknown voltage is to be measured. The membrane is driven with an A.C. reference voltage and brought near the wire carrying the unknown D.C. voltage, thereby generating an alternating attractive and repulsive force caused by electrostatic attraction between the membrane and the wire with the D.C. voltage. The force applied to the membrane has two basic frequency components, and causes the membrane to move and thereby move the microphone diaphragm to which the membrane is attached. The motion of the microphone diaphragm causes the microphone (or appropriate transducer) to produce an electrical A.C. output voltage containing the same frequency components that the alternating forces contain. This microphone output has two primary frequency components, the first of which is at the same frequency as the reference voltage, and the second of which is at double this frequency. The first component is proportional to the unknown voltage being measured times the known reference voltage times a constant proportional to the coupling capacitance between the wire with the unknown voltage and the membrane. The second component is proportional solely to the known reference voltage squared times the coupling capacitance between the wire and the unknown voltage and the membrane. By dividing the amplitude of the first component by the amplitude of the second component, an accurate determination of the unknown voltage may be made, regardless of changes and uncertainties in the coupling capacitance that may be due to differences in wire geometry, variations in positioning of the wire with respect to the sensing diaphragm, differences in dielectric constant, and other factors which can affect coupling capacitance. Moreover, despite the fact that we are measuring a D.C. voltage, the microphone and all subsequent electronics operate solely with A.C. voltages, eliminating errors due to low frequency vibration, temperature drift, 1/f noise, gravitational effects, and other error sources which plague prior art D.C. voltage measurement apparatus. In addition to measuring the D.C. component of an unknown voltage, the present invention can measure all A.C. components up to one half of the highest frequency to which the microphone or other transducer is capable of responding.
Measurements of voltages on several conductors may be made by duplicating the apparatus described above. The technology of the invention can be expanded to imaging of the voltage on complex electrical structures, through the use of arrays of sensors. In this way, two-dimensional images of the potentials of traces and components on a printed circuit board may be obtained. By combining this novel non-contacting voltage measurement technique with conventional D.C. current measurement apparatus, it is possible to perform fully non-contacting power measurements. Accurate measurements may be made on both insulated and bare conductors, due to the ability to eliminate the effects of differing capacitance values between the conductor and sensing membrane, by means of dividing the amplitude of the first frequency component by that of the second frequency component.